Kootenai River Fishery Updates

Didymosphenia geminata (Didymo)

Didymosphenia geminata (Didymo) is an aquatic stalked diatom that was first documented in the Montana portion of the Kootenai River in 1977, but it wasn’t until the early 2000s that this algae species garnered public attention when it began developing dense seasonal mats in the Kootenai River. Montana Fish, Wildlife and Parks (FWP) is concerned about Didymo because it has the potential to impact the food web of the Kootenai River by reducing the available space for certain macroinvertebrates which are preferred trout food. FWP first started monitoring didymo in the Kootenai River in 2009 immediately downstream of Libby Dam where these mats were especially dense. The increases in these dense Didymo blooms also coincided with a precipitous decline in rainbow trout numbers in the Kootenai River immediately downstream of Libby Dam. Although trout anglers on the Kootenai River in 2012 and 2013 generally saw good action, Montana FWP biologists were concerned enough to launch a detailed study to learn more about factors that contribute to growth and survival of trout in the river.

These circumstances prompted FWP and the US Army Corp of Engineers to collaborate on a research project beginning in 2013 aimed at taking a closer look at the factors responsible to the prolific blooms of Didymo and investigate if anything can be done to control this undesirable species of algae in the Kootenai River. The US Army Corp of Engineers constructed an experimental flume system on the north bank of the Kootenai River in late 2012 at the USGS gauging station that uses water pumped out of the Kootenai River to grow Didymo. FWP funded a three year graduate student to conduct experiments using the flume system to assess the feasibility of adding various substances to limit the growth of Didymo. The graduate student successfully completed three years of research, much of which focused on the role that nutrients play in limiting the severity of Didymo growth. Perhaps the most important finding from this research was the discovery that very low levels of phosphorous tend to cause Didymo to produce the thick stalk portion in excess and stalk production is further enhanced when the nitrogen to phosphorus ratio strongly favors nitrogen. FWP is continues to evaluate the efficacy of the newly developed Didymo management tool. The next step in the process will be to conduct a small scale in-river pilot study to ensure that the results from the graduate student research that were conducted in artificial toughs are applicable to the Kootenai River and to further estimate how large of and impact Didymo may be having on the Kootenai River trout population.

In addition to funding the research to investigate potential control measures of Didymo in the Kootenai River, FWP is also conducting applied research to estimate the impacts that Didymo is having on the growth and survival of trout in the Kootenai River. The multi-year study that began in 2011 annually marks a total of approximately two thousand trout in four sections of the Kootenai River with passive integrated transponder (PIT) tags. The PIT tags used by Montana FWP are essentially the same microchips that veterinarians put in dogs and cats. Each PIT tag allows researchers to identify individual fish which will be used to determine growth and survival estimates throughout the Montana portion of the Kootenai River below Libby Dam. Marked fish may not be easily identified by the casual angler since the tags are imbedded in the flesh behind the dorsal fin. However, all marked trout have also had the adipose fin clipped. The adipose fin is the small fleshy fin directly in front of the tail on the top of the trout’s back.

Information obtained from this study may be used to: provide water management recommendations, serve as a basis for considering alternative fishing regulations, and guide restoration activities on the Kootenai River. The study results and recommendations will be available in 2018.

Figure 1. Photograph of the experimental flume system constructed on the west bank of the Kootenai River below Libby Dam. This picture was taken December 2012 during construction of the site.

Figure 2. Photograph of the replicated experimental flumes used to grow Didymosphenia geminata and test the feasibility of developing a potential control mechanism that could be applied to the Kootenai River at a later date to limit the distribution and abundance of this species of algae. This picture was taken prior to the beginning of the experiments conducted during 2013.

Figure 3. Close up photograph of four of the sixteen experimental flumes used during the 2013 experiments conducted at this site. Experiments in 2013 tested addition of 3, 5, and 8 micrograms of phosphorous as a potential control mechanism for Didymo. Other algae blooms of green filamentous and other (non-didymo) diatom blooms occurred in all three flumes treated with phosphorous. The control (third flume from the left) did not experience large algal blooms and was dominated by Didymo.

Photograph of informational signs Montana Fish, Wildlife and Parks placed at common fishing access locations on the Kootenai River below Libby Dam.